Character generation

ABSTRACT

A system for high-speed scanning of graphical symbols in which a line is scanned by a first waveform at a first speed and upon the scan passing over the leading or trailing edge of a symbol, an output is developed and applied to a pulse generator. The pulse generator produces a substantially rectangular pulse whenever the scan passes a leading or trailing edge of a symbol portion, while the pulses are integrated by an appropriate amplifier. The integrated pulses form a second triangular-shaped waveform which is combined with the first scanning waveform such that the scan speed of the first waveform is temporarily reduced below normal scan speed and is temporarily increased above normal scan speed before reaching and remaining at a normal speed. The combined scanning waveform is then transmitted to a display device along with the first waveform and appropriate control signals such that the symbol scanned may be remotely displayed.

United States Patent 3,402,258 9/1968 Lerner 3,410,953 11/1968Quinlan.... 3,479,453 1 l I969 Townsend Priority Gordon HughesManchester, W

Aug. 21, I968 Aug. 3, 1911 hm Computers Limited hudon, W

Aug. 26, 1967 Great Brink lnventor Assignee cnmc'rsn GENERATION 6Claims, 2 Drawing Figs.

Refierences Cited UNITED STATES PATENTS l78l DlG. 3 l78/DIG. 3 l78/6.8

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osc-- FOREIGN PATENTS 546,462 7/1942 Great Britain 17 8/6 794,531 5/1958Great Britain l78/DlG. 3

Primary Examiner- Robert L. Griffin Assistant ExaminerJoseph A. Orsino,J r. Anomey--l-lane & Baxley ABSTRACT: A system for high-speed scanningof graphical symbols in which a line is scanned by a first waveform at afirst speed and upon the scan passing over the leading or trailing edgeof a symbol, an output is developed and applied to a pulse generator.The pulse generator produces a substantially rectangular pulse wheneverthe scan passes a leading or trailing edge of a symbol portion, whilethe pulses are integrated by an appropriate amplifier. The integratedpulses form a second triangular-shaped wavefonn which is combined withthe first scanning waveform such that the scan speed of the firstwaveform is temporarily reduced below normal scan speed and istemporarily increased above normal scan speed before reaching andremaining at a normal speed. The combined scanning waveform is thentransmitted to a display device along with the first waveform andappropriate control signals such that the symbol scanned may be remotelydisplayed.

CHARACTER GENERATION BACKGROUND or THE INVENTION The scanning ofcharacter or symbol outlines to generate I electrical signals which areused to control a cathode-ray tube in such a way as to display acorresponding character or symbol outline is well known. In presentdevices, it is desirable to scan symbols as rapidly as possible formaximum efficiency. However, it will be apparent that an increase in thenumber of characters scanned per second requires a correspondingincrease in the rate of scanning each character and, therefore, in thehigh frequency components of the resulting electrical signals. One ofthe problems .encountered in such systems is that electrical signalsmust be fed from the scanning system to the display system over someform of transmission channel. However, the bandwidths of thetransmission channels, if the channels are of any substantial length,will not effectively permit the transmission of such high frequencysignals and, will be in many cases the factor which limits the maximumscanning rate. Also, the bandwidths of video amplifiers employed todrive bright up circuits of the display device limit the practical rateof scan. Therefore, bandwidth reduction of electrical signals generatedfrom the scanning of symbols has been employed to allow the effectivetransmission of such signals to a display device.

Various prior art systems have been developed in which bandwidthreduction techniques are employed for narrowing the band of transmittedfrequencies in such characterscanning arrangements. For example, onesuch technique is shown in British Pat. No. 659,596, which shows atwo-speed scanning method. In this patent, a line-scanning technique isemployed such that the area of a television screen is scanned at a highrate and, upon the scan passing an image, the scan rate is reduced. Asthe scan line covers the image and encounters a blank area again, thescan speed is returned to a rapid rate. As noted in this patent, aconsiderable reduction of the transmitted frequency band representativeof the image is obtained as a result of scanning the image at a slowerrate. However, in the subject patent, a comparison between the intensityof blank" and image areas must be effected before the scan is switchedto a slower rate, the switching operation also requiring externalcircuitry.

SUMMARY According to the present invention, a system for scanninggraphic symbols is provided wherein a first scanning waveform is adaptedto scan a line across a symbol at a first speed with the passage of suchscan across a symbol detected by suitable means. First and secondsignals corresponding to the position of the scan across the leading andtrailing edges of the symbol, respectively, are generated and applied tosuitable circuit means for producing second waveforms. The secondwaveforms are combined with the first scanning waveform such that thescan speed of the first waveform is temporarily reduced below the normalspeed and then temporarily increased above normal speed as a result ofthe scan passing each one of the symbol portion edges. Thus, the firstand second pulses generated each effectively both reduce and increasethe speed of the first scanning waveform.

In the present invention, the normal scanning speed along a line isslowed down as a result of the scan encountering an edge of a symbol,with the scan speed subsequently increased to a rate greater than thenormal scanning speed. Both the slow and high scan rates are temporarywith the high scan rate lasting until the normal scan rate is reached.At this point, the normal scan speed is continued until the second edgeof the symbol portion is encountered. At this second edge, the scan rateis temporarily reduced and increased as previously described. Changes inthe scan rate, as mentioned above, are

effected by adding or combining second waveforms with the main scanningwaveform such that the symbols are rapidly scanned, yet efficientlytransmitted to a remote display device. It should be noted that whilethe present invention provides a system for rapid scanning of graphicsymbols, the outlines of such symbols are scanned at a substantiallylower speed, thereby allowing effective transmission of electricalsignals representative of the symbol outlines.

As a result of altering the main scan rate by combining a secondwaveform therewith, various complex switching circuits have beenrendered unnecessary. Also, while it is known to determine scan rates bythe discharge of a capacitor in a resistor-capacitor discharge circuit,the need to switch appropriate resistors, and the attendant problems ofsuch switching circuits, is eliminated in the present invention.

BRIEF DESCRIPTION OF THE DRAWING the system of DESCRIPTION OF THEPREFERRED EMBODIMENTS Electrical signals representing symbols aregenerated by a conventional monoseope character generator tube 1 (FIG. 1The screen of the tube carries secondary emission areas corresponding tothe shapes of symbols, such as alphabetic characters. The monoseopeelectron beam is scanned in a line across the screen by the applicationof scanning waveforms to deflection electrodes 2 and 3. When theelectron beam strikes a portion of a secondary emission area, an outputvoltage is developed across resistor 4. The monoseope tube is providedwith a conventional deflection system for deflecting the beam in adirection perpendicular to that deflection produced by the electrodes 2and 3. This further deflection system is not shown, since it is notnecessary to an understanding of the invention.

The scanning waveform which is applied to the deflection electrode 2 isderived from an oscillator 5, operating at a frequency of 1 MHz., forexample. The waveform may be of a conventional sawtooth form,.or it maybe sinusoidal, the substantially linear part of the sine wave beingeffective for scanning the beam across the screen.

The output voltage across resistor 4 is applied to the input of a videoamplifier 6,-having a limited bandwidth of 3.3 MHz. in the presentexample, and a pulse generating circuit 7. The passage of the monoscopebeam across a portion of a symbol area produces a substantiallyrectangular pulse, as shown by waveform B (F IG. 2). The pulsegenerating circuit 7 produces a pair of pulses (waveform C) in responseto the rectangular input (waveform B), the edges of the pair of pulsescoinciding with the leading and trailing edges of the input pulse. Thismay be done, for example, by differentiating the input pulse and usingthe resulting pulses derived from the leading and trailing edges of theinput pulse to trigger a monostable delay line pulse generator (notshown). The pair of pulses are of equal duration, 50 nsec. durationbeing suitable in the present example. The output of video amplifier 6is shown in waveform E of FIG. 2. The bandwidth of video amplifier 6 isdirectly determined by the time necessary for the edges of waveform E tosettle with this time increment (50 nsec.) being equal to the durationof the pulses of waveform C.

The pulse pair is fed to an amplifier circuit 8 which integrates eachpulse to form a substantially triangular pulse of nsec. duration(waveform D). The output waveform from the amplifier 8 is applied to thedeflection electrode 3. Waveform A illustrates the net deflectionvoltage applied to the monoseope tube, which is the resultant ofcombining the linear waveform =from oscillator 5 with the waveform D.The effect of the additionof waveform D is to reduce and then increasethe rate of change of the scanning voltagefor each character edge whichis scanned. Hence, the rate of scanning is first reduced and thenincreased as compared with the average scanning rate produced by themain scanning waveform from the oscillator 5. The pair of pulses(waveform C) are slightly delayed relative to the waveform B so as toallow the leading edge of this waveform to stabilize before the waveformD is applied to the electrode 3. As stated previously, while waveform B(FIG. 2) is a substantially rectangular pulse and the edges of the pulsewill not be completely vertical, the pulse will have a finite rise timewhich, as is the case with conventional monoscope tubes, will depend onthe size of the scanning spot and on the particular materials comprisingthe secondary emission areas of the tube 1. While the production ofwaveform D by pulse generator 7 and integrating amplifier 8 will requirea finite time delay, pulse generator 7 will respond to the voltageacross resistor 4 and may produce waveform B before the exact edge of asymbol portion is scanned as a result of the scanning spot having afinite size etc. Thus, by the time waveform D is applied to deflectionplate 3, the main scanning waveform produced by oscillator 5 may beconsidered as crossing the actual" edge of the character portion.Similar considerations will apply to the trailing edge of waveform B.

The signals from the amplifier 8, the oscillator 5, and the videoamplifier 6 are fed over a cable 9 to control a remote displaycathode-ray tube 10. The scanning waveforms from the amplifier 8 and theoscillator 5 are fed, via current drivers 1 l and 12, respectively, toelectromagnetic deflection coils l3 and 14, respectively. The responsecharacteristics of the drivers 11 and 12, together with those of thecoils l3 and 14 are such that the electron beam of the cathode-ray tubescans in synchronism with the beam of the monoscope 1. The output of thevideo amplifier 6 is fed to control grid 15 of the cathode-ray tube 10.The control grid is normally biased to such a level that the CRT beam iscut off and the signal from the video amplifier acts as a bright up"waveform to bring the control grid above cutoff. Hence, when themonoscope beam scans a portion of a character, a corresponding brightimage is produced on the CRT screen. The amplitude and shape of thewaveform D is such that the scan produced by the main scanning waveformis first substantially cancelled and then increased to twice the averagespeed of the main scanning waveform. It will be appreciated that theamplitude and/or shape of the wavefonn D may be such that the changes inthe scan rate due to scanning an edge are less than that shown and thatthe changes may be nonlinear, for example, they may be exponential.

It has been assumed that the width of the portion of the character whichhas been scanned is such that the variations in scanning rate due to thetrailing edge commence immediately after the variations due to theleading edge. if the width of the character portion is less than that ofthe example, the waveforms produced by successive edges will overlap tosome extent This can result in a reduction of the amplitude of the netscanning voltage, if several portions of a character occur in one scan.This reduction may be compensated by increasing the amplitude of themain scanning waveform from the oscillator 5. Such an increase in theamplitude of the main scanning waveform will result in less of a slowingof the scanning speed as the addition of waveform D (FIG. 2) have asmaller effect in slowing down the scanning speed and a greater effectin increasing the scan speed. Therefore, the scanning speed will berestored to an original rate sooner, and thereby allowing smallercharacter widths to be scanned. If the width of the character portion isgreater than that of the example, the

variations in scanning rate due to the leading and trailing edge of thecharacter portion will be separated by an interval during which scanningis at the normal rate. Consequently, there will be a variation inbrightness across the character portion displayed on the cathode-raytube. In order to produce a uniform brightness a suitable compensatingwaveform is added to the si nal passing through the video amplifier.

he invention is applicable to forms of character scanning other than themonoscope. For example, the characters may be apertures in a mask andthe scanning waveforms may be used to control the beam of a cathode-raytube to produce a spot of light which scans across the mask. The lightwhich passes through the apertures in the mask is picked up by anoptical system and a photoelectric cell, the output from whichcorresponds to the output from the monoscope. Alternatively, thecharacters may be recorded as images on a photographic film or printedoutlines on paper.

I claim:

1. A system for scanning graphic symbols including means for generatinga first scanning waveform which is adapted to scan a line across asymbol at a first speed, means for detecting the passage of the scanacross a portion of the symbol, means for generating first and secondpulses corresponding to the portion of the scan across the leading andtrailing edges of said symbol portion, respectively, means responsive tosaid first and second pulses to generate second scanning waveforms andmeans to combine said first and second waveforms to cause the scanningspeed along said line to be reduced temporarily to less than said firstspeed and subsequently increased temporarily to a speed greater thansaid first speed as a result of said scan passing each one of said edgesof said symbol portion.

2. A system as claimed in claim 1 in which said means responsive to saidfirst and second pulses includes an integrating amplifier operative tointegrate said first and second pulses and thereby produce said secondwaveforms, each of said second waveforms having a substantiallytriangular waveshape.

3. A system as claimed in claim 2 in which said means to combine saidfirst and second waveforms includes means to add the first half of eachof said second waveforms to said first waveform to temporarily reducethe scanning speed below the speed of said first waveform and to add thesecond half of each of said second waveforms to said first waveform totemporarily increase the scanning speed above the scanning speed of saidfirst waveform.

4. A system as claimed in claim 2 in which said means for detecting thepassage of a scan across a portion of said symbol includes an electrontube having a first deflecting means to which said first waveform isapplied anda second deflecting means, the output of said tube being to apulse generator for producing said first and second pulses that areintegrated by said amplifier to produce said second waveforms, and meansto apply said second waveforms to said second deflecting means so thatsaid first and second waveforms are combined.

5. A system as claimed in claim 4 in which the output of said electrontube is applied over a transmission means to a remote display device fordisplaying said symbol scanned by the combined first and secondwaveforms.

6. A system as claimed in claim 5 in which said display device includesa second electron tube having third and fourth deflecting means, saidfirst and said combined waveforms applied over said transmission meansto said third and fourth deflecting means, respectively, and means forapplying a control signal over said transmission means to a controlelectrode of said second electron tube.

1. A system for scanning graphic symbols including means for generatinga first scanning waveform which is adapted to scan a line across asymbol at a first speed, means for detecting the passage of the scanacross a portion of the symbol, means for generating first and secondpulses corresponding to the portion of the scan across the leading andtrailing edges of said symbol portion, respectively, means responsive tosaid first and second pulses to generate second scanning waveforms andmeans to combine said first and second waveforms to cause the scanningspeed along said line to be reduced temporarily to less than said firstspeed and subsequently increased temporarily to a speed greater thansaid first speed as a result of said scan passing each one of said edgesof said symbol portion.
 2. A system as claimed in claim 1 in which saidmeans responsive to said first and second pulses includes an integratingamplifier operative to integrate said first and second pulses andthereby produce said second waveforms, each of said second waveformshaving a substantially triangular waveshape.
 3. A system as claimed inclaim 2 in which said means to combine said first and second waveformsincludes means to add the first half of each of said second waveforms tosaid first waveform to temporarily reduce the scanning speed below thespeed of said first waveform and to add the second half of each of saidsecond waveforms to said firsT waveform to temporarily increase thescanning speed above the scanning speed of said first waveform.
 4. Asystem as claimed in claim 2 in which said means for detecting thepassage of a scan across a portion of said symbol includes an electrontube having a first deflecting means to which said first waveform isapplied and a second deflecting means, the output of said tube being toa pulse generator for producing said first and second pulses that areintegrated by said amplifier to produce said second waveforms, and meansto apply said second waveforms to said second deflecting means so thatsaid first and second waveforms are combined.
 5. A system as claimed inclaim 4 in which the output of said electron tube is applied over atransmission means to a remote display device for displaying said symbolscanned by the combined first and second waveforms.
 6. A system asclaimed in claim 5 in which said display device includes a secondelectron tube having third and fourth deflecting means, said first andsaid combined waveforms applied over said transmission means to saidthird and fourth deflecting means, respectively, and means for applyinga control signal over said transmission means to a control electrode ofsaid second electron tube.